The invention relates to a charge air cooler for internal-combustion engines, having at least two cooling circulations which are guided through heat exchanger blocks which are connected behind one another in the flow direction of the air and have different coolant temperatures.
Charge air coolers for internal-combustion engines are known (German Patent Document DE OS 23 42 787). Because, in the case of such charge air coolers, the entering air has very high temperatures, there is the danger that the heat exchanger elements situated on the inlet sidexe2x80x94as a rule, therefore tubes equipped with finsxe2x80x94are very highly thermally stressed and therefore may be damaged, because erosion caused by cavitation phenomena may occur, in addition to considerable thermal expansions.
In the case of the above-mentioned known charge air coolers, it was attempted to solve this problem in that the heat transfer on the first rows of tubes was kept as low as possible but, in the rows of tubes which follow, the heat exchange was increased to the optimum. It was endeavored to achieve this either in that the number of fins per tubes was selected to be rising in the direction of the flowing-through air, or in that the first row of tubes was constructed without fins or was even provided with a heat insulation. Charge air coolers of this type therefore require very high expenditures. At the very high entry temperatures of the charge air of up to 300xc2x0 and even higher which are becoming increasingly customary, such measures are either no longer sufficient or result in charge air coolers whose dimensions are very large. This also applies to charge air coolers according to German Patent Document DE 196 44 584 A1 (corresponding to U.S. Pat. No. 5,845,701), in which it was attempted to reduce the thermal stress to finned tube blocks of charge air coolers in the area of the lateral parts in that the outermost corrugated fin layer, which rests against the lateral parts, was in each case displaced against the charge air flow over the entry edge of the finned tube block toward the front into the entry area for the charge air formed by the lateral parts and the radiator tanks. Although, as a result, a faster warming-up of the lateral parts can be achieved, the problem of occurring erosion in the entry area cannot be easily solved in this manner.
From German Patent Document DE 41 14 704 C1, it is known to carry out the cooling of charge air in two stages. There, the cooling first takes place in a high-temperature charge air cooler which is connected with a high-temperature intercooler, and then in a low-temperature charge air cooler which is situated in a circulation with a low-temperature intercooler. These two charge air coolers are then followed by an engine oil cooler and a transmission oil cooler. The charge air cooler therefore corresponds to the initially mentioned charge air cooler which is characterized by at least two cooling circulations which are guided through heat exchanger blocks connected behind one another in the flow direction of the air. These two cooling circulations have different coolant temperatures. However, possibilities of designing the high-temperature cooler such that it will permanently withstand the high entry temperatures without increasing the dimensions, are not mentioned there.
It is therefore an object of the present invention to construct a charge air cooler such that charge air of very high temperatures can be cooled at economically acceptable expenditures, without the occurrence of the above-mentioned problems.
For achieving this object, it is suggested according to the invention that, for a charge air cooler of the initially mentioned type, at least the heat transfer block provided as the first heat transfer block in the flow direction of the air be produced of a more erosion-resistant and temperature-stable material than the heat transfer blocks which follow and is situated in a high-temperature coolant circuit in which the coolant temperature is selected to be as high as possible.
As a result of this measure, it becomes possible to provide on the entry side of the charge air a front-end cooler which, from the beginning, is designed such that first the very high charge air temperatures are cooled down to an acceptable degree, so that then the heat transfer blocks which follow can be designed according to the known criteria. Thus, a coolant temperature can, for example, be provided for the front-end cooler which reduces the temperature gradient between the entering charge air and the coolant temperature. Because the front-end cooler is also made of a more temperature-stable material than the heat transfer blocks which follow, it becomes possible to largely eliminate the risk of damage at the entry of the charge air.
In another embodiment, it can be provided that the first two heat transfer blocks of three heat transfer blocks are situated in a high-temperature coolant circulation and only the last heat transfer block is situated in a low-temperature coolant circulation. The temperature of the coolant used in the first two coolant circuits can be relatively high at the inlet side, in which case the flow of the coolant will be selected such that, in the first heat transfer block, a cooling of the hot 300xc2x0 charge air is achieved to values around 180xc2x0 C. and, in the second heat transfer block, to values of a magnitude of 110xc2x0 C.
In such an embodiment, the forward flow temperature of the coolant in the third heat transfer block can amount to approximately 45 to 50xc2x0 C., as in conventional coolers, and the air can be cooled to approximately 60xc2x0 C. or slightly above that.
As an embodiment of the invention, the first heat transfer block can be a circular-tube cooler made of steel, of a copper zinc alloy or of an aluminum alloy. In contrast, the second and the third heat transfer block can be made in a conventional stacked construction of aluminum elements, as described, for example, in German Patent Document DE 196 44 586 A1. Finally, all heat transfer blocks can be combined to a charge air cooler unit which, as a whole, is installed in corresponding facilities or vehicles, but is supplied by different coolant circuits.